1. Field of the Invention
The present invention relates to a first in first out hydration tank that prevents liquid flowing through the tank from stagnating in certain areas of the tank, thereby facilitating flow that is truly first in first out through the tank.
2. Description of the Related Art
In gel mixing systems, it is desirable to have first in first out hydration tanks so that gel mixtures flowing through the tanks have consistent and predictable residence time within each tank. Even if a traditional hydration tank has a defined flow circuit provided through the tank, such as the three tanks taught in FIG. 2 of Applicant's U.S. patent application Ser. No. 10/426,742, there can still be a problem when the fluid that is flowing through the tanks is a highly concentrated fracturing gel mixture.
The reason this is true is that fracturing gel made from guar is a non-Newtonian fluid. Newtonian fluids, such as water and oil, will flow whenever even a slight pressure is applied to the fluid. Non-Newtonian fluids, on the other hand, require that a certain threshold pressure be applied to them before they begin to flow. This is due to the yield point of the fluid. Thus, non-Newtonian fluids have a threshold pressure required to start them moving, and below which they may deform but will not move. This phenomenon is referred to as gel strength and is directly proportional to the force required to cause the fluid to start moving.
In the mixing system described in Applicant's U.S. patent application Ser. No. 10/426,742, a concentrated gel is prepared that can have significantly higher viscosity and gel strength than that of the final product. The concentrate allows greater hydration time in limited tank volume but has the problem of higher viscosities and gel strengths in the mixing and the hydration tanks.
If the fluid is not managed properly, parts of the tank will become gelled and motionless and will be difficult to get moving again. When gelation occurs, the objective of first in first out flow is defeated because the gelled fluid will remain in one place and the newly mixed fluids that enter the tank will bypass the gelled fluid. Thus, the tank is functionally smaller than its actual size since part of the fluid in the tank is not moving.
The present invention addresses this problem by stirring the fluid as it passes through the hydration tank, thereby preventing dead spots within the tank. By providing mixing that is normal to the nominal direction of flow, i.e. not forward or backward relative to the direction of flow through the tank and providing shear within virtually all of the volume, the mixing prevents the occurrence of dead spots or channeling within the flow path, while not moving some of the liquid towards the discharge port faster than other parts of the fluid volume, thereby insuring that all the fluid ends up with exactly the same residence time in the tank. By employing mixing that is normal to the flow of the liquid through the tank, all of the fluid flow paths through the tank move at a uniform velocity.
While the fluid is moving though the hydration tank, it is continuing to hydrate and thus continuingly increasing in viscosity. If the fluid does not keep moving uniformly through the hydration tank, it is possible that some parts of the fluid in the tank would develop greater viscosity due to slower velocity through the tank and therefore greater residence time. The slower moving volume within the tank will continue to develop higher viscosities which in turn tends to further slow its movement until eventually it could stop moving and become gelled. Once gelled, a much greater force is required to get the gel started moving again.
The present invention keeps all of the fluid moving at a uniform velocity so that there will not be areas with higher or lower viscosity at the same position within the flow path. Although viscosity will increase due to hydration from the entrance of the present tank to the exit, all fluid that is at the same position relative to the entrance and exit of the tank should have the same viscosity.
Still a further object of the present invention is an output from the tank that is uniform in its level of hydration. That is possible only if all of the liquid moves through the tank at the same velocity.